Conditions of a Substance on its Absorption of Light. 625 



to an increase in the displacement of the absorption lines 

 towards the region of longer wave-length. This is Kundt's 

 result. 



Also according to Wood there appears to be evidence that 

 the position of an absorption band depends on the concen- 

 tration of the solution. This, of course, is exactly analogous 

 to the case discussed in the previous paper. An increase in 

 the density of the absorbing electrons displaces the line pro- 

 portionately to the red end of the spectrum. 



2. Absorption in a gas. — It is generally assumed that the 

 absorption produced by a given quantity of gas is independent 

 of its density. The important discovery was, however, made 



by Angstrom that the compression of a gas increased the 

 intensity of its absorption, the mass of gas traversed re- 

 maining the same. The general results of this and much 

 subsequent work of a similar kind are summarized by Wood 

 in the four following statements : — 



1. Beer's law does not in general hold for gases. 



2. If to a gas at a definite volume we add a foreign gas 

 which dues not act on it chemically, its absorption is 

 increased. 



3. The absorption of a gas mixture is greater than the 

 sum of the absorptions of its separate constituents each one 

 taken at its partial pressure. 



4. The absorption of a gas mixture is, on the contrary, 

 equal to the sum of the absorptions of its constituent parts if 

 each part is taken at a pressure equal to the total pressure. 



Now equation (i.) above shows that the maximum ab- 

 sorption coefficient is 



. fh. ■ . 1 



** 2W (1-aA) 2 



and results (1) and (2) above are direct deductions from this 

 formula. The simple statement contained in (4) does not 

 appear, however, to be exact ; although it is probably 

 approximately true for the case of gases. 



It is important to notice that the results of this and the 

 last paragraph are simply two aspects of the one general 

 problem. In the case of liquids the point investigated 

 appears to have been the displacement of the absorption 

 lines, their intensities being left out of consideration. In the 

 case of gases it appears to have been the alteration in the 

 intensity which first attracted notice. The two phenomena 

 should, however, be present in both experiments; and the 

 theory is equally valid for the two cases. . . 



